The present invention relates to apparatus for automatically limiting the cathode ray tube beam currents in a video display system and has particular utility when used in association with a projection television system.
Television receivers frequently include circuits for limiting cathode ray tube beam currents to prevent damage such as phosphor burnout and aperture mask distortion resulting from the intense heat generated by excessive current levels. Typically, prior art beam current limiting circuits operate by developing a signal which is proportional to the total beam current, i.e. the sum of the individual beam currents produced by each of the three electron guns of the cathode ray tube, and using the developed signal to adjust the gain and/or drive of the video processing unit of the receiver. Therefore, in response to the development of an excessive total beam current, the gain or drive of the video processing unit is automatically reduced thereby reducing each of the individual beam currents by a proportionate amount. The signal proportional to the total beam current is normally developed by sensing the high voltage anode current supplied to the cathode ray tube or the low voltage current supplied to the high voltage horizontal output transformer. Prior art circuits exemplary of the foregoing are disclosed in U.S. Pat. Nos. 3,842,201, 3,924,067, 4,079,424, 4,096,518, 4,137,552 and 4,207,591.
In the prior art automatic beam current limiting circuits, each electron gun of the cathode ray tube may be operated for producing the maximum allowed total beam current. That is, if, for example, the circuit is set for initiating limiting action at a threshold total beam current of one milliampere, when a pure green field is being displayed by the cathode ray tube, the green gun will be allowed to produce one milliampere of beam current before the limiter becomes effective. Likewise, when a pure blue or pure red field is displayed by the cathode ray tube, the blue and red electron guns could be operated for producing one milliampere beam currents. Of course, if more than one of the electron guns where being operated at any given time, the threshold total beam current of one milliampere would be shared among the operating guns prior to initiation of the limiting action. While these operating characteristics produce satisfactory results in conjunction with a conventional television receiver, certain problems arise when an attempt is made to incorporate such beam current limiting circuits in, for example, projection television receiver systems.
More particularly, projection television systems commonly employ three separate cathode ray tubes each having a plastic lens associated therewith for focusing either green, red or blue light onto an image display screen. In order to prevent damage to the lenses, the heat associated with the light beams developed by each respective cathode ray tube must be limited to a value corresponding to a predetermined beam current level, this predetermined level typically being on the order of about 400 microamperes. Thus, the maximum beam current in each of the three cathode ray tubes must be limited to a value of about 400 microamperes. At the same time, however, in order to provide an acceptably bright white field image display and to take advantage of the maximum capability of the sweep and high voltage system, the total beam current should preferably be allowed to assume a value substantially greater than the foregoing predetermined level, for example, up to about one milliampere. Accordingly, in order to prevent damage to the focusing lenses while, at the same time, providing an acceptable white field image display, the beam current produced by each respective cathode ray tube must be limited to a value of about 400 microamperes with the total beam current being allowed to assume values of up to about one milliampere, this value being determined for any particular receiver by the limitations of the horizontal sweep transformer. It will be appreciated that the prior art total beam current sensing circuits discussed above are inherently incapable of achieving this result. In particular, if the circuit is set for initiating limiting action at a total beam current of 400 microamperes to correspondingly limit the beam current produced by each respective cathode ray tube to protect the associated focusing lenses, then an acceptable white field image cannot be produced. On the other hand, if the circuit is set for initiating limiting action at a total beam current of one milliampere to provide an acceptable white field image, then inadequate protection is provided for the focusing lenses since each individual beam current could assume the one milliampere limiting level.
Accordingly, it is an object of the present invention to provide an improved automatic beam current limiting circuit which is particularly useful in association with a projection television receiver.
Another object of the invention is to provide an automatic beam current limiting circuit for limiting the beam currents in each cathode ray tube of a projection television receiver system to a predetermined level while allowing the total combined beam current to assume a value substantially larger than the predetermined level.